The description of the basis of the claim for the connection between the republican constitution and the end of perpetual peace is made in a brief paragraph. The basis of it turns on the appeal to consent that we found integral to the very notion of republicanism (and one of the features that we indicated distinguished it from liberalism). For a republican state to wage war it requires the consent of its citizens. This basic fact of a republican state is indicated by Kant to predispose a republican state to peace. The rationale for this claim is that war is, as Kant puts it, a "bad game" (Ak. 8: 350). There are inevitable hardships suffered in the case of war including the fact that the citizens of the state will themselves have to fight the war (this surely intimates a view of the army as voluntary and not a "standing" one). Similarly, the financing of the war will be based on the contributions of the citizenry (through taxes). Finally, the financing will carry over into the eventual peace through the countries' indebtedness.
By contrast, in a constitution in which the subjects are not citizens (i.e. a non-republican constitution) the head of state is intrinsically distinct from the other members of the state having no representative connection with them. As such the head of state can treat his subjects in this case as mere mechanisms for his own ends, substituting these ends of his own for publicly available ones hence indicating that a non-republican constitution is intrinsically out of harmony with respect for humanity.
The argument for republican peace thus turns on the claim that if a citizenry that is engaged in its country in such a way that the country is one that it clearly "owns" then it will be cautious in endangering it. The reason for this will be that such a citizenry puts its own property at risk in engaging in war. The ruler in the non-republican constitution does not put his property at risk in any particular sense and relates to his subjects as means to his own ends so is at war with them. Such a ruler is thus merely re-directing his general policy of war when he engages in conflict with other states, not altering the overall policy as would occur for a republic.
Assumptions that guide this argument are that a republic would not wish to "out-source" the conflicts it engaged in, by, for example, employing mercenaries to fight its wars for it or by having a principled distinction between a standing army and the ordinary citizenry. Both these points are clearly built in to the general claim of the basis of the republic as one that possesses an active citizenship. The point concerning the army would imply, however, either a general absence of an army except in cases of specific conflict (surely a foolhardy policy) or a general citizen army based on continued engagement (something like a general policy of enlistment or conscription for limited periods so that the entire citizenry is engaged in defence). The latter view cuts against "liberal" assumptions of the correct way to run a state in peace time and could be modified to allow citizenship engagement (so not necessarily military but this one amongst the options much like with Obama's recent notion of voluntary engagement).
Either way the view of citizenship involved in the idea of the republic appears rather more active than is generally given in most existent models of the state (except perhaps in a country that is constantly ready for war such as Israel). However, without some assumption of this kind it is less obvious why a country with a republican constitution would be peaceful. Current advocacy of the related idea of "democratic peace" suggests different reasons for peaceful behaviour and will be examined in a subsequent posting.